In the semiconductor industry, a photolithography method using visible light or ultraviolet light has been employed as a technique for writing, on a Si substrate or the like, a fine pattern, which is required for forming an integrated circuit comprising such a fine pattern. However, the conventional photolithography method has been close to the resolution limit, while microsizing of semiconductor devices has been accelerated. In the case of the photolithography method, it is said that the resolution limit of a pattern is about ½ of an exposure wavelength, and that even if an immersion method is employed, the resolution limit is about ¼ of an exposure wavelength. Even if an immersion method using an ArF laser (193 nm) is employed, it is estimated that the resolution limit is about 45 nm. From this point of view, EUV lithography, which is an exposure technique using EUV light having a shorter wavelength than ArF lasers, is considered to be promising as an exposure technique for 45 nm or below. In this specification, “EUV light” means a ray having a wavelength in a soft X-ray region or a vacuum ultraviolet ray region, specifically a ray having a wavelength of from about 10 to 20 nm, in particular, of about 13.5 nm±0.3 nm.
EUV light is apt to be absorbed by any substances and the refractive indices of substances are close to 1 at this wavelength, whereby it is impossible to use a dioptric system like a conventional photolithography employing visible light or ultraviolet light. For this reason, for EUV light lithography, a catoptric system, i.e. a combination of a reflective photomask (EUV mask) and a mirror, is employed.
A mask blank is a laminate for a photomask, which has not been patterned yet. In the case of an EUV mask blank, it has a structure wherein a reflective layer for reflecting EUV light and an absorber layer for absorbing EUV light, are deposited in this order on a substrate made of glass or the like (Patent Document 1). Besides these layers, in such an EUV mask blank, a protection layer for protecting the reflective layer at a time of forming a mask pattern in the absorber layer, is usually deposited between the reflective layer and the absorber layer. Further, on the absorber layer, a low reflective layer is usually deposited for improving the contrast at a time of inspecting a mask pattern.
In such an EUV mask blank, the thickness of the absorber layer is preferably thin. In EUV lithography, exposure light is not incident from a perpendicular direction to an EUV mask but incident from a direction at an angle of a few degrees, usually 6 degrees, to the perpendicular direction. If the thickness of the absorber layer is thick, at a time of EUV lithography, a shadow of the exposure light is formed on a mask pattern formed by removing a part of the absorber layer by etching, and the pattern accuracy or the dimension accuracy of a mask pattern (hereinafter referred to as “transfer pattern”) transferred to a resist on a substrate such as a Si wafer by using the EUV mask, tends to be deteriorated. Since this problem becomes more significant as the line width of the mask pattern formed on the EUV mask becomes smaller, the thickness of the absorber layer of the EUV mask blank is preferably thinner.
For the absorber layer of the EUV mask blank, a material having a high absorption coefficient for EUV light is employed, and the thickness is ideally a thickness whereby EUV light incident into a surface of the absorber layer is completely absorbed. However, as described above, since the thickness of the absorber layer is required to be thin, it is not possible for the absorber layer to completely absorb EUV light incident into the layer, and a part of the light becomes reflected light.
In the step of forming a transfer pattern of a resist on a substrate by EUV lithography, an important characteristic is the contrast of reflected light from the EUV mask, that is, the contrast between reflected light from a portion of the mask wherein the absorber layer is removed at a time of forming the mask pattern so that the reflective layer is exposed to the outside, and reflected light from a portion of the mask wherein the absorber layer is not removed at the time of forming the mask pattern. Accordingly, it has been considered that so long as a sufficient contrast of reflected light is obtained, there is no problem even if the incident EUV light is not completely absorbed by the absorber layer.
Based on the above concept, in order to reduce the thickness of the absorber layer, an EUV mask using the principle of phase shift is proposed (refer to Patent Document 2). This has a characteristics that a portion from which the absorber layer is not removed at a time of forming a mask pattern has a reflectivity of from 5 to 15%, and that reflected light from such a portion has a phase difference of 175 to 185° from reflected light from a portion where the absorber layer is removed at the time of forming the mask pattern so that the reflective layer is exposed to the outside. The document describes that with the EUV mask, by using the principle of phase shift for reflected light from the absorber layer, it is possible to maintain a sufficient contrast to the reflective layer, and accordingly, it is possible to reduce the thickness of the absorber layer.